Land Disturbing Activity (LDA) and Stormwater

Updated: June 2020


This website includes information on drainage, grading and land disturbing activity requirements and includes links the drainage manual, code, checklists, and assistance bulletins.

The County's current stormwater and drainage requirements include:

Note: The County is currently reviewing its regulations for a drainage regulation update to be effective on July 1, 2021. You can review the proposed updates on the 2020 NPDES page

In addition to the permit or land use application submittal requirements (e.g., single-family home, private garage/shop, land disturbing activity, short plat, or subdivision), documentation is required for compliance with drainage requirements.

Drainage minimum requirements vary depending on the scope of the proposed project and on the physical and environmental conditions on the site where the new development activity is proposed. Key considerations include:

  • The quantity of hard surface to be added or replaced;
  • The extent of land disturbing activity (clearing or grading);
  • The size of areas to be converted from vegetation to lawn, landscaping or pasture;
  • Location relative to floodplains or shorelines, steep slopes, and potential impacts to wetlands, streams, waters of the state or natural drainage courses; and
  • Drainage from the site into the municipal separate storm sewer system (MS4)

Based on these key considerations, your project may be:

  1. Exempt from drainage requirements (see SCC 30.63A.200);
  2. Qualify for an exception from certain drainage requirements (see SCC 30.63A.210 and Bulletin #107);
  3. Require compliance only with minimum requirement 2 to prepare a stormwater pollution prevention plan (SWPPP) per SCC 30.63A.445 - this could be either a small SWPPP (see SCC 30.63A.810) or a full SWPPP;
  4. Require a targeted stormwater site plan and compliance with minimum requirements 1 through 5;
  5. Require a full stormwater site plan and compliance with minimum requirements 1 through 9

Use Figure 1.1 and 1.2 in Volume I, Chapter 2 of the Drainage Manual to help determine what minimum requirements (MRs) are applicable to your project; see also SCC 30.63A.300 or 30.63A.310 as applicable for new development or redevelopment projects.


The following assistance bulletins are good resources for questions related to Land Disturbing Activity (LDA) Permits and drainage review for proposed projects:

The federal Clean Water Act (CWA) includes a permit program, the National Pollutant Discharge Elimination System (NPDES) that accounts for stormwater runoff that collects in municipal separate storm sewers (MS4s) and enters surface waters. The Environmental Protection Agency (EPA) developed rules to implement the regulations and established two phases, Phase I and Phase II, for the municipal stormwater permit program. The Washington State Department of Ecology develops and administers the NPDES municipal stormwater permits in Washington State. For more information, please visit the Washington Department of Ecology website.

For more information about Low Impact Development (LID) and training opportunities, Please visit our LID webpage for resources and information on Low Impact Development.

Earlier versions of the Stormwater, drainage, and land disturbing regulations can be found below:

Requirements September 30, 2010 - January 21, 2016

Requirements prior to September 30, 2010